A liquid crystal display (LCD) is a device that uses the light modulating properties of liquid crystals (LCs) to selectively filter incoming light to produce black and white or color images. An LCD is generally made up of a layer of LC molecules aligned between two transparent electrode layers (indium tin oxide (ITO)), two glass substrates and two polarizing filters (front and back polarizers, sometimes including retardation films), where the axes of light transmission of the polarizers are arranged perpendicular to each other (e.g., in the case of a “twisted-nematic” LCD). The surface of each of the electrode layers that is in contact with the LC molecules is treated (e.g., with a thin polymer layer that is unidirectionally rubbed, also known as an alignment layer) so as to align the LC molecules in a particular direction in the absence of an applied voltage. The direction of the LC alignment is then defined by the direction of rubbing.
There are many types of LCDs, using different types of LC materials and different orientations. The LC molecules are often oriented or aligned in a helical or “twisted” direction in the absence of an applied voltage. In the “active driving” context, LCDs are typically twisted-nematic LCDs (e.g., TN type TFT LCDs) in which the twisted angle between the LC molecules in contact with the alignment layers is 90 degrees. In the “passive multiplex driving” context, LCDs are either twisted-nematic LCDs for use in passive low multiplex driving situations (e.g., less than about 16 multiplexes) or “super twisted-nematic” LCDs (e.g., STN LCDs) for use in high multiplex driving situations (e.g., greater than or equal to about 16 multiplexes). For STN LCDs, the twisted angle is generally greater than or equal to 180 degrees, and the first polarizing axis may not necessarily be perpendicular to the second polarizing axis.
In any case, a reflective surface or, less commonly, a backlight, is arranged behind the back polarizer. Images are produced by the LCD when light is transmitted from the backlight through both of the back and front polarizers. As the light passes through the LC material it can be selectively rotated in a polarization orientation (or not rotated) so that the light is selectively blocked by (or passes through) the front polarizer. Further, the LCD is typically divided into separate portions, known as picture elements (or pixels). To turn off a pixel of an LCD (which can be thought of as a shutter) so as to prevent the transmission of light through the front polarizer, a voltage is applied across the LC material of the pixel which changes the orientation of the LC molecules making up the pixel (shutter) by causing the LC molecules to align themselves with the electric field instead of being aligned in a helical orientation. Depending on the orientation of the LC molecules, the polarized light passing therethrough is either passed without changing the polarization orientation of the light, or the polarization orientation is rotated 90 degrees. In one type of display, each pixel can be paired with a colored filter (to form a single sub-pixel) to remove all but the red, green or blue (RGB) portion of the light from the original white light source. The sub-pixels are so small that when the display is viewed from even a short distance, the individual colors blend together to produce a single spot of color, a pixel. The shade of color is controlled by changing the relative intensity of the light passing through the sub-pixels.
FIG. 1 illustrates one type of LCD device 10 including an LC material 14 that is operable to selectively allow light to pass through the device 10. The device 10 includes first (e.g., front) and second (e.g., rear) ITO layers 18, 22 (each of which includes a plurality of electrodes) that are spaced apart by conductive spacers 26 and sealed adjacent their outer perimeters by any appropriate seal frit 30 (e.g., seal glue). The conductive spacers 26 are electrically interconnected to conductive traces (not shown) of both of the first and second ITO layers 18, 22. Although at least the majority of the seal frit 30 may be allowed to harden in any appropriate manner (e.g., drying, curing), an aperture (not shown) may be formed or left in the seal frit 30 to allow for the introduction of the LC material 14 into the space between the first and second ITO layers 18, 22. First and second polyimide alignment layers 34, 38 having first and second respectively grooved surfaces (not labeled) are respectively disposed on inside surfaces of the first and second ITO layers 18, 22 so as to align the molecules of the LC material 14 in the direction of the grooved surfaces in the absence of an applied electric field. One or more additional spacers 42 may be included to space apart the first and second polyimide layers 34, 38. First (e.g., front) and second (e.g., rear) transparent plates 46, 50 (e.g., glass or plastic plates) are arranged parallel to each other and disposed on outside surfaces of the first and second ITO layers 18, 22. With reference to FIG. 1, the portion of the device 10 spanning from the first transparent plate 46 towards and including the second transparent plate 50 may be considered an “LCD cell” 100.
The device 10 also includes first (e.g., front) and second (e.g., rear) polarizers 54, 58 (e.g., linear polarizers with retardation films) arranged parallel to each other and disposed on outside surfaces of the first and second transparent plates 46, 50 of the LCD cell 100. For instance, the polarization axes of the first and second polarizers 54, 58 may typically be aligned orthogonally with respect to each other. The device 10 also includes at least one conductive connector 60 (e.g., flexible printed circuit (FPC) connector) and a driver 62 that may be electrically interconnected to the device 10 at any appropriate location (e.g., as shown, to the second transparent plate 50) for applying a voltage to the LC molecules of various pixels or sub-pixels of the device 10 (via the electrodes of the first and second ITO layers 18, 22) to position the LC molecules in a particular orientation. More specifically, the driver 62 serves to flow a current through conductive traces of the second ITO layer 22 which current passes through the conductive spacers 26 to the conductive traces of the first ITO layer 18. While not shown, the driver 62 may be directly electrically interconnected to the conductive traces of the second ITO layer 22 or else directly electrically interconnected to one or more intermediate conductive traces or wires that are directly electrically interconnected to the conductive traces of the second ITO layer 22. Additionally, the device 10 includes any appropriate backlight (not shown) that operates to transmit light through the second polarizer 58 towards the first polarizer 54.
The polarizers 54, 58 serve to filter the transmitted light so that the light passes therethrough in only one plane or orientation of polarization. Thus, light beams are transmitted or blocked depending upon the position of the polarizers 54, 58 with respect to one another and the voltage applied via the electrodes of the ITO layers 18, 22 to the LC material 14, with the result that a corresponding driven pixel of the display appears dark or bright (and a non-driven pixel is the opposite). In this regard, images may be displayed on the device 10 by selectively controlling the brightness of each pixel.
One primary weakness of the device 10 is the low response time of the LC material 14 at low temperatures. At temperatures below −30° C., for instance, the device 10 can cease refreshing of the display. Also at such temperatures, the display contrast and viewing angle can drop to unacceptable levels. The main reason for these drawbacks is the increased viscosity of the LC material 14 at such low environment temperature which limits the orienting effect that an applied voltage would otherwise have on the molecules of the LC material 14. Additionally, the birefringence of the LC material 14 (i.e., the decomposition of a light ray into two rays when it passes through the LC material 14) in combination with cell gap thickness also will be changed at such low operating temperatures, which impacts display transmittance, contrast and viewing angle.
A popular method for remedying the above disadvantages is attaching a glass ITO heater under the device 10 to heat the LC material 14 when the LC material 14 is working at low environmental temperatures. Turning to FIGS. 2a-2b, a standard structure of a glass ITO heater 200 is illustrated. The heater 200 includes a glass plate 204, an ITO layer 208 disposed over one surface of the plate 204, and a pair of connectors 212 (e.g., FPC connectors) electrically connected to electrodes of the ITO layer 208 at opposing ends of the ITO layer 208. By applying a voltage across the ITO layer 208 and passing a current therethrough, the heater 200 generates heat by resistive heating of the ITO layer 208. With additional reference now to FIG. 3, a device or assembly 300 is formed by attaching the heater 200 to the LCD device 10. Specifically, the ITO layer 208 of the heater 200 is interconnected to the second polarizer 58 of the device 10 via a pair of pieces or strips of doubled-sided tape 216. In the interest of clarity, the LC cell 100 has been largely represented in FIG. 3 as a schematic box (with only the first and second glass plates 46, 50 being shown) and the connectors 212 of the glass ITO heater 200 have been removed.
As shown, an air gap 220 naturally exists between the ITO layer 208 of the heater 200 and the second polarizer 58 due to the use of the double-sided tape 216. A backlight 224 is provided to transmit light through the glass plate 204 of the heater 200, the second polarizer 58, etc. so as to produce an appropriate image with the device 10. Any appropriate controller or driver may pass an applied voltage via the connector 212 to the electrodes of the ITO layer 208 to produce heat that is transmitted towards and through the second polarizer 58, the second transparent plate 50, the second ITO layer 22 and the second polyimide layer 38, and eventually to the LC material 14 to reduce the viscosity thereof.
The inventor has determined that utilizing a glass ITO heater to heat LC material in the manner discussed above includes a number of shortcomings that limit the ability of the heater to effectively reduce the viscosity of the LC material and/or limit the performance of the assembly 300. In one regard, the air gap 220 between the heater 200 and the device 10 causes light reflection and corresponding light transmission loss as light transmitted from the backlight 224 passes into the air gap 220 towards the second polarizer 58. Furthermore, the thickness of the air gap 220 is often uneven which increases the likelihood of “Mura” defects (i.e., irregular luminosity variation defects) in resulting images. Still further, light rays traveling from the backlight 224 can experience low-levels of birefringence as they travel through the glass plate 204.
In addition to the adverse effects on light transmission, the above manner of heating the LC material 14 is inefficient as the generated heat must travel through a number of layers (e.g., the second polarizer 58, the second transparent plate 50, etc.) before eventually reaching the LC material 14. Furthermore, the glass plate 104 of the heater 100 is unnecessarily thick and susceptible to breakage, and the resistance of the ITO layer 108 may be affected over time due to oxidation and exposure to air (e.g., via the air gap 120).